Sure using this will just send a bunch of gibberish to Gmail in the body such that Gmail won't be able to auto-scan for ad keywords to send to you. But this probably will be used by .000001% of Gmail users, so I doubt they are worried about this affecting their business in any meaningful way.
And, even among those people who do use it, a big percentage of their email will remain unencrypted. Transactional email like flight or hotel booking confirmations sent from the airline or hotel. I have to think that's the valuable stuff to Google in terms of selling ads, not the topics you would choose to encrypt.
So the downside to them is not that big. And it's offset by the upside of user trust and confidence being maintained, or at least eroding less.
